Optical frequency modulation (OFM) is believed to be a preferred format for signals carried in free-space optical links. When compared with the conventional technique of amplitude modulation (AM) and direct detection, the signal-to-noise ratio achieved with OFM is better by the amount (fFM/Δf)2, where fFM is the Frequency Modulation (FM) index and Δf is the maximum modulation frequency (or the signal bandwidth). The benefit of OFM is especially important for applications in which the optical power incident on the receiver station is weak. This would typically be the case for a free-space optical link, which achieves power-efficient performance.
Typically, OFM is achieved by directly modulating the current driving a semiconductor laser, which results in both amplitude modulation and frequency modulation of the output. See FIG. 1(a). Any residual amplitude modulation (RAM) of a frequency-modulated (FM) laser will degrade the signal-to-noise ratio of a communications link or degrade the sensitivity of a remote sensing apparatus. This is because that amplitude modulation is indistinguishable from added noise.
Typical communication signals, radar signals and channelized wideband electronic warfare signals have a bandwidth as high as 1-2 GHz. Thus, it is desirable to have a large FM Index and a maximum frequency excursion that may be in excess of 10-20 GHz. One example of a state-of-the-art device is described in IEEE J. Sel. Topics Quantum Electronics V.1, pp.461-465 (1995). This device could serve as the Master Oscillator (MO) used in the disclosed embodiments of the present invention. The FM efficiency achieved is approximately 1 GHz/mA and decreases with higher average drive current. This device has RAM that results in an intensity modulation of approximately 0.18 mW/GHz. A maximum frequency excursion in excess of 80 GHz was obtained as well as a flat modulated-frequency response to 20 GHz. This device is suitable for most FM applications. However, even for a maximum frequency excursion of 10 GHz, and an average output power of 7 mW (which is approaching the limit of the maximum frequency response), the RAM is 1.8/7 or 0.26. Thus, the SNR of an OFM Link incorporating this laser cannot be better than 6 dB. The RAM produced by this FM laser must be suppressed or dampened in order to achieve a higher SNR. As an alternative to direct modulation of the laser, a separate, external modulator is usually recommended for OFM transmitters, to achieve lower RAM. A phase modulator is often used, with the phase modulation rate corresponding to the amount of frequency modulation desired. However, that approach makes the OFM transmitters and the drive electronics for those transmitters more complicated.
Typically, a high-power OFM transmitter is obtained by amplifying the output of a single OFM laser, generally with an optical-fiber amplifier. This amplifier can produce a maximum output level of 20-25 dBm, but faithfully amplifies the RAM fed to it from the FM laser. The amplifier also adds even more amplitude noise in the form of relative intensity noise (RIN).
Optical injection locking of an array of slave lasers (SL) to a single master oscillator (MO) has been reported by many investigators. One example is given in Appl. Phys. Letters, v.50, pp. 1713-1715 (1987). In much of the known work, both the MO and the SLs are operated CW (continuous wave), with DC drive currents. The goal of the injection locking is generally to achieve mutually phase-locked operation of the multiple SLs and thereby obtain a diffraction-limited, high-power output beam from the array of SLs. Use of a frequency-modulated MO to injection lock multiple SLs is apparently unknown.
Optical injection locking of different SLs to different modulation sidebands of an amplitude-modulated MO is described in J. Lightwave Technol., v.6, pp. 1821-1830 (1988). In this work, each SL is tuned to have a different free-running wavelength that is matched to a different modulation sideband of the MO output. Although the MO is amplitude modulated, there is some accompanying frequency modulation. In contrast to the present invention, wherein each SL output is at the same wavelength, the outputs of the SLs in this prior-art approach are at different wavelengths. These wavelengths can be used for different WDM channels or pairs of those wavelengths can be used to generate RF signals, by means of optical heterodyning.
OFM can be used in a number of applications, including communication applications. For example, free-space optical links with OFM are useful for both terrestrial and inter-satellite communications. A typical wavelength for such communication links is 1550 nm.
OFM can be used in other applications, including remote sensing applications. For example, an ultra-sensitive method for detecting trace amounts of chemical or biological compounds is FM-DIAL or frequency modulation spectroscopy. This method requires a frequency-modulated optical source whose wavelength corresponds to the wavelength of the chemical feature being detected. Such wavelengths could cover the range from ultra-violet to the far infrared. Residual amplitude modulation could reduce the sensitivity of this technique, although techniques could be developed to compensate digitally for that effect since the RAM is predictable. Also, for this application, a portion of the transmitted signal can be made available to the receiver for coherent detection and processing.
Remote sensing can be used to detect pollutants and biological and/or other materials harmful to human beings or other life forms and to detect other chemicals of interest in a myriad of applications. The basic architecture involves a single Power Oscillator (PO) or a combination of multiple power oscillators arranged in parallel so that their optical outputs are combined.
In terms of the remote sensing art, the prior art includes modulated power oscillators, which are limited in sensitivity by RAM. RAM can corrupt the desired signals to be processed. As indicated above, the presence of the species to be detected is inferred by its effect on the amplitude and phase of the ensemble of sidebands, which can be compromised by RAM. Since, in the prior art, the same diode laser is used as the modulation source as well as for the generation of high output power, system tradeoffs are inevitable in terms of output raw power, generation of the required spectrum (high-index FM modulated sidebands) and low RAM. The present invention enables one to address all these metrics separately in the system by optimization of the modulation parameters at the MO Level, and power scaling at the SL level.
In the prior art (see FIG. 1(a)), a single diode laser has been employed for remote sensing, using a modulated electrical current supply to frequency modulate the transmitter source. However, its power output scalability and sensing range are constrained since diode lasers have maximum output powers on the order of about 500 mW. (For remote sensing applications at infrared wavelengths, quantum cascade lasers may be employed, typically owing to their desirable output wavelength range and tunability. However, such lasers have somewhat lower maximum output powers.) In addition, the ultimate sensitivity is limited in this prior art system due to RAM on the output signal.
The prior art depicted by FIG. 1(a) shows a power oscillator (PO) that is a directly modulated diode laser. As pointed out above, this approach demands that a single device be simultaneously optimized in terms of its output spectral purity, low residual amplitude modulation (RAM), high modulation index (required for robust FM modulation spectroscopy) and high optical output power, with thermal loading and optical damage of the components as constraints. Such an approach requires that engineering tradeoffs be made amongst these parameters. Moreover, in order to scale such an approach to still higher powers, separate power supplies are required for the modulation, which increases the size, weight, cost, and complexity of the scaled system. Moreover, in the prior art, as more POs are added for scaling of the output power upwards, any phasing of the wavefront produced by the combination of POs must be accomplished at the high-power optical output ports, thereby placing even greater demands on the power handling capability and size of the phase shifters, etc., as illustrated in FIG. 1(b).
Thus, there is a need for an approach to produce an effective amplification of the FM laser output while suppressing that laser's RAM. The present invention achieves that goal. Furthermore, even higher output level can be attained with the present invention because it combines the outputs of multiple SLs, each of which can have an output level that exceeds 20 dBm.
This invention can generate a high-power frequency modulated optical beam with reduced residual amplitude modulation. The invention preferably includes a frequency modulated master oscillator (MO) that optically injection locks multiple slave lasers (SLs), which receive constant (DC) current drives. The outputs of the multiple SLs are then combined into a single output optical beam using known techniques. Since the multiple SLs are injection locked to the same MO, they are frequency locked and thus phase locked (but not necessarily in phase) with each other. Because the gain of each SL is preferably clamped, residual amplitude modulation of the injection source (the MO) is overcome by the clamping. High output power is achieved because the optical injection ratio can be quite low (10−3 to 10−4). Thus, one MO can be used to couple a large number (>100) of SLs. In addition, each SL can be a high-power device (with an output exceeding 100 mW) whereas the MO can be a lower power device that is selected for its frequency-modulation properties. The phase of each SL output depends on the wavelength detuning between the wavelength of the MO and the free-running wavelength of that SL, which can be adjusted by its drive current and/or temperature or by other means such as controlling the length of the optical path of each SL. Thus, the relative phases of the multiple SL outputs can be controlled and this is preferably done by controlling the electrical drive currents of each SL independently. The relative phases of the SLs can be set to achieve beam shaping or steering.
The maximum frequency modulation (FM) bandwidth of this transmitter is limited by the injection-locking bandwidth of the SL. This locking bandwidth is typically 1-2 GHz (half bandwidth). Another embodiment places each SL in a homodyne phase-lock loop that compares a portion of the outputs of the MO and that SL to generate an electrical error signal for the SL. By combining optical injection with the phase-lock loop, the injection locking bandwidth can be increased by about an order of magnitude, to beyond 10 GHz.
It is well known that the RAM of a frequency modulated diode laser can be mitigated by using that laser as a MO to injection lock a single subsequent SL. An analysis of this RAM mitigation is described in J. Lightwave Technology V.16, pp. 656-660 (1998). According to this article, a MO injection-locks a single SL as is illustrated by FIG. 1(c). The MO is modulated (modulation examples are discussed below) and it, in turn, drives the SL. For an arbitrary modulation format, the function of the SL is to intensify the MO signal, while preserving the modulation encoding, which is typically a high-index FM format with many narrow-linewidth sidebands (see FIG. 1(a)). A key advantage of this architecture is that there is minimal RAM by virtue of the injection-locking approach relative to other approaches (e.g. MOPAs, direct modulation of power oscillators).
As is disclosed herein, the OFM transmitter can be scaled upward in power by using the MO to drive a set of POs in parallel, or, in another embodiment, a cascade architecture can be employed that utilizes the MO, followed by an intermediate PO, which, in turn, injection locks yet another set of higher power POs in parallel. Each PO is a slave laser to the PO or MO that precedes it. Since the injection-locking architecture provides for both frequency as well as phase-locking of the PO to the MO, precise overall wavefront control of the system can be realized by using optical phase shifters placed between the MO and each of the POs (assuming single-transverse-mode operation of the POs), so that minimal side lobe structure and speckle result from the composite system. Modulation techniques include current modulation of the MO and the use of external modulators, such as optical waveguide phase modulators and optical MEMS-based phase modulators. Since the modulation is performed at a low power portion of the system, a variety of compact, low-voltage and low-power-consuming integrated optical modulators can be used, which would otherwise be impractical for use in high-power laser oscillators. The system is robust; if a given PO fails the system experiences graceful degradation. Finally, only one compact modulator is required, which resides in the low power portion of the system (internal to the MO or external to the MO). This is the case even as the system is scaled in total power, as opposed to the need for a set of separate modulators to service each of the POs (with the concomitant high-power handling requirements), as in the prior art.